DESCRIPTION Males are more susceptible to many parasite infections than females. The goal of this research proposal is to determine whether males are more susceptible to viruses, specifically hantaviruses, because androgens suppress immune function or because males engage in more androgen- dependent behaviors (e.g., aggression) that influence susceptibility to hantavirus. These studies are based on field observations of many species that report a high proportion of males being infected with hantavirus. The goal of this proposal will be met by examining: the role of sex steroid hormones in sex differences in immune responses to hantavirus infection; 2) how androgen metabolites influence the course of hantavirus infection; 3) when during ontogeny hormones influence adult susceptibility to infection; 4) whether aggressive males are more susceptible to hantavirus infection than less aggressive males. These studies represent a thorough examination of potential mechanisms that underlie population variation in hantavirus infection and serve to expand our knowledge of disease processes in general and, specifically, factors that affect susceptibility to hantavirus.